Magnetic recording and reproducing



July 4, 1950 H. a. SHAPER ET AL 2,513,683

MAGNETIC RECORDING AND REPRODUCING Filed March 19, 1946 s Shee ts-Sheet 1 uo-z 1 -7 7 D B I OUTPUT INVENTORS HARRY B. SHAPER zmxygy B. MILLER ATTORNEY y 50 H. B. SHAPER ET m. 2,513,683

MAGNETIC RECORDING AND REPRODUCING Filed March 19, 1946 3 Sheets-Sheet 2 July 4, 1950 H. a. SHAPER El AL ucwnc RECORDING AND REPRODUCING 3 Sheets-Sheet 3 .\lll ll I6 IZZVEENTORS M ,d BY

' Patented July 4," 1950 MAGNETIC RECORDING AND BEPRODUCING m a. Shaper and Harry B. Miller, Cleveland, Ohio, assignors to The Brush Development 00., Cleveland, Ohio, a corporation of Ohio Application March 19, 1946, Serial no. 655,448 1 Claim. (emu-95) The present invention relates to improvements in apparatus for recording transient signals for later observation, and is more particularly con-' cerned with improvements in systems in which the-transient signals to be observed are recorded upon an endless recording medium by the use a carrier wave which is frequency-modulated by the transient signals, this record being used to actuate an indicator through the intermediary of a pick-up and a frequency discriminator.

According to the present invention, improved and novel means are provided for recording just the correct interval of the transient corresponding to one complete impression upon the endless record. Also, according to the present invention, novel apparatus is provided for converting the transient signal to be recorded into the frequencymodulated signal which is directly recorded upon the endless record. As a further feature of the invention, improved and novel frequency discriminator apparatus is provided for converting the frequency-modulated signal picked up from the record into a faithful reproduction of the desired transient signal.

The various objects and advantages of the present invention will be best understood from the following description of a preferred exemplification thereof, reference being had to the accompanying drawings, wherein Fig. l is a schematic block diagram of the entire system of the present invention.

Fig. 2 is a schematic wiring diagram of the modulated oscillator; trigger control circuit and obliterating oscillator of Fig. 1.

Fig. 3 is a similar wiring diagram of the discriminator of Fig. 1.

Fig. 4 is a similar wiring diagram of a modifled discriminator circuit useful in place of that of Fig. 3. e

The system of the present invention is shown in Fig. l. The transient signal is arranged to be recorded upon an endless recording medium ifll which is shown, by way of example, as in the form of a magnetic tapeor wire which is continuously impelled to travel over two rollers ilk-5 at constant speed by any suitable means, such as a. synchronous motor (not shown). In this figure it is assumed that the tape is impelled in a clockwise direction, as indicated by the arrows. Mounted adjacent to and in successive operative relation with the recording medium Iii-4 are a recordinghead l0'3, an obliterating head III-4i and a pickup or reproducing head [0-6, the obliterating head |0--|l being placed adjacent the recording head "-3 to act upon a portion of the tape IH very soon after the record is impressed thereon, the pickup head ill-4 being spaced widely from both the other heads Iii-4| and i03.

Coupled to the recording head iU-J is a frequency-modulated oscillator i02 which, in the absence of a signal to be recorded, feeds a continuous constant-frequency carrier signal, such as 01' the order of 10 kilocycles per second in frequency, into the recording head ill-4, by which this constant-frequency signal is recorded upon the tape ill-4 in well-known fashion.

Coupledto the obliterating head ill-H is an obliterating oscillator Ill-40 which normally continuously supplies oscillations to the obliterating head i0H of a frequency suitable for obliterating the record on the tape 10-4 just previously made by the recording head ill-4. As

an illustration, the obliterating frequency may.

be about 30 kilocycles per second. In this way the 10-kilocycle signal recorded on the tape in the absence of an input signal is continuously wipedofi or obliterated from the record almost immediately.

When a signal to be recorded (such as a tram sient) is impressed upon the input circuit i0i, it serves to frequency-modulate the output of oscillator ill-4 in accordance with the amplitude or the transient, so that the record impressed on the tape i0l by the recording head i0--3 is no longer of a constant frequency, but varies in frequency in accordance with the transient signal. At the same time, this input transient signal is supplied to a trigger control circuit Ill-9 connected to the obliterating oscillator iii-40, and serves to interrupt or cut oif the oscillations of the obliterating oscillator. This action is'made more rapid than the time required for any particular portion of the tape to travel from the recording head to the obliterating head, so that by the time that the portion or the tape upon which is recorded the frequency-modulated signal corresponding to the transient arrives at the position of the obliterating head, that head is no longer energized, and the record is accordingly not obliterated.

The trigger control circuit i0-9 is also connected to the modulated oscillator Ill-2 and acts to cut oil the oscillator and thereby halt the recording operation after a time interval equal to the time taken for the tape to make one complete revolution. Inthis way the transient is recorded just for the interval of one rotation of the tape and no longer.

The frequency-modulated oscillations recorded on the tape are picked oil by the pickup head ll-l and are supplied to the discriminator "-1 which converts them into a replica of the original input transient signal. Since the tape is endless and is continuously impelled, the output or the discriminator is repeated cyclically at the ire-=- quency o! rotation of the tape which, for example, may be 10 cycles per second. This cyclic signal may be visually indicated byimpressing it upon a pair deflecting plates of a cathode ray indi= cater or oscilloscope it-t upon whose other deflecting plates is impressed a ten-cycle synchonlzed time-sweep. In this manner the transient signal is reproduced as a stationary indication upon the screen of the oscilloscope.

The specific details or one form of transient= recording circuit exemplifying the invention are shown in Fig. 2, wherein the circuit is subdivided by three horizontal dashed lines into four sections: namely, section Et-i comprising the elements of the frequency-modulated oscillator 96-2; section 2@-2 comprising the amplifier and rectifier stages of the trigger control circuit 95-9; section Eli-t comprising the trigger stages of circuit iii-9; and section it-o comprising the elements of the obliterating oscillator iii-i s.

The frequency-modulated oscillator section 28-8 includes the tubes 52 and it forming the oscillator. Tube it has a cathode resistor lb and acts as a cathode follower, its cathode being connected to the control grid of tube 52 througha phase-shifting circuit or network comprising serice-connected capacitors i6, ii, is, as and shuntconnected resistors 2b, 2i,22 and 23. Tube it has a plate resistor i i, and its anode is connected to the control grid of the tube i8. Tube it also has a cathode resistor 2d shunted by a by-pass capacitor 25. Tubes is and i2 therefore form a phase-shift oscillator whose frequency is determined by the phase delay in the capacitor resistor network iii-23. The alternating component of the voltage appearing across the oathode-follower resistor to is supplied to the input of an amplifier or power output tube to through coupling capacitor 28 and a voltage-dividing cir= cuit comprising resistors 29 and iii. The output oi power tube at appearing across its plate resistor it is supplied to the recording head ill-3 through a coupling capacitor 35.

In such an oscillator. oscillations are generated at the frequency at which the capacitor-resistor network provides a phase delay of 180. If the tallies of capacitance or resistance are varied so as to tend to change this phase delay, the frequency of the oscillator will correspondingly vary to restore the phase delay to 180 at the new frequency. Accordingly, by controlling the resistance values in accordance with a modulating signal, frequency modulation of the output of the oscillater can be obtained. In the present system, tubes 3'5 and as provide such control.

Each of these tubes to and 3? acts as a variable resistance in series with the respective resistors 2c and M. Referring specifically to tube 35, it will be seen that this tube is a double triode having two cathode-anode paths in series. The resistor is is connected to ground through the lower triode section, and the control grid of this triode section is supplied in the manner described below with the modulating signal. The control grid of the upper triode section is maintained at a fixed potential by means of voltage divider 48. In this manner the resistance in series with resistor is directly controlled by the modulating signal while maintaining fixed potential iii upon the plate of the lower triode section, which is connected directly to resistor 2d. Tube 3? has exactly the same circuit as tube 85, and operates in the same manner.

The transient signal to be recorded is supplied to the input terminals id-i and thereby to the voltage divider 88 which permits adjustment of the amplitude of the modulating signal applied to the control tubes 36 and ill. The bias for these tubes is supplied from a voltage divider 5i through a coupling resistor The signal derived from voltage divider $38 is fed to the lower grids of tubes 36 and 3? through several resistor-capacitor net-= works.

The first network deilned'by resistors st, to and capacitors cc and' ifi serves to give frequency compensation for the remaining portions of the systern. This network, together with the network formed by resistor ti and capacitor 38, are designed as a filter, which prevents oscillator frequency energy from being impressed upon the control grids of the control tubes to and 3?, without materially impeding the passage of signals whose frequency is lower than that of the oscillator frequency. This is necessary, since any oscillator frequency energy inserted into the phase=shift network 16-28 would disrupt the desired linear relation between output frequency and input modulating signal.

Where, as here, two or more separate branches or the phase-shift network have their resistances varied by means of control tubes with their control grids excited in parallel by the same modulating signal, it is also necessary to decouple these control grids with respect to oscillator frequencies, since the several control tubes operate in branches of the phase-shift network which have difierent amplitudes and phases. Thus, any

' coupling which would occur between the various control grids of the control tubes would tend to upset the oscillation frequencies and their linear relationship to the modulating voltages. This decoupling is produced by the resistors 62 and capacitors 33, which eliminate oscillation frequency energy from the control grids of the two control tubes and thereby decouple these grids from one another at the oscillator frequencies.

in Figure 2 only two resistance legs of the phase-shift network are shown controlled by control tubes. However, all four of these resistance legs, or more, should more legs be used, may be arranged to be controlled by similar control tubes, where, for example, wider swings of oscillator frequency are required.

In operation, in the absence of input modu lating signals, the oscillator i2, is continuously supplies energy of its mean or carrier frequency to the recording head As discussed above, these oscillations are recorded upon the tape iii-=43, but are almost immediately obliterated by.

the obliterating head it--i i. The obliterating oscillator iiii ii, which supplies the excitation for the obliterating head, is shown in section 26-43 of Figure 2 and comprises an oscillator tube at couplied to a power amplifier 98, which, in turn, is

coupled to the obliterating head iii-ii. The bias on the grid of the obliterating oscillator tube 532 is controlled in the manner described below. so that the obliterating oscillations may be out ofi under proper conditions.

As is discussed above, the present system operates to control the obliterating oscillator to cut out its obliterating action immediately upon coencement of recording of an input transient usual It also stops the recor action after one complete rotation of the tape subsequent to the initiation of recording of a transient signal,

tube 89 by way of a resistor-capacitor network 54. This network 64 forms a low-pass filter circult, preventing any very high frequency components in the transient input signal from being supplied to tube 69. This assures that the trigger amplifier 60 will respond only to desired signals.

The output of the amplifier tube 60 is applied to a phase-inverting amplifier 6I. Phase inverter 6| has its output connected to the balanced rectifier stage 19 serving as a full-wave rectifier and having a single load resistor 99.

In the balanced rectifier 19, when either control grid is supplied with a positive signal, an output voltage appears across the load resistor 80. In this way the circuit 29-2 responds to the input transient signal to provide a positive voltage across resistor 80 no matter whatypolarity the input signal may have.

The trigger circuit shown in section -3 comprises a grid-controlled gaseous rectifier 8|, such as of the Thyratron type. The plate of the tube Si is connected to a source of positive potential through resistors 82, 83, and its cathode is connected to ground through resistor 81.

The normal potentials upon the electrodes of tube 8i are adjusted so that the tube is normally non-conductive. However, when a suflicient positive voltage is impressed upon the control grid by resistor 89 in response to he input transient signal, tube 3| becomes suddenly conductive. When this happens, the potential of the junction 89 of resistors 82, 83 is suddenly lowered. This junction 89 is connected through a resistor 9! to the control grid of the obliterating oscillator 93. Accordingly, the bias of the obliterating oscillator control grid is thereby reduced to the point that the obliterating oscillator is cut oil, and obliterating action is halted.

ofl' of the obliterating oscillator occurs in less time than that required for a particular element of the tape to travel from the recording head to the obliterating head. In this way, obliterating action is halted, as required for proper recording of the desired transient signal.

When tube BI becomes conductive, the poten tial of its cathode rises suddenly, becomin positive. This positive potential is supplied to the control grid of a similar grid-controlled gaseous rectifier 96 through a time-delay circuit composed of resistors 99 and capacitor 95. The time delay interposed by this resistor capacitor network 94, 95 is selected to be substantially equal to or slightly less than the time required for an element of the tape to make one complete rotation around the loop of the tape.

The plate of gas tube 96 is connected to the positive potential source through resistors 99, 99. Its cathode is connected to the cathode of the modulated oscillator tube l2, and thence to ground through the resistor 24. Thus, when tube 96 becomes suddenly conductive, after the tape has been completely recorded upon, the current through resistor 24 abruptly rises. This increases the negative bias upon the control grid of oscillator tube I2 and cuts off the oscillator, whereby 6 oscillations are no longer supplied to the record ing head III-3. Thereafter, the recording tape may be continuously rotated, and only the pickup head is eflective, so that the transient signal recorded during the single rotation of the tape may be viewed upon the oscilloscope or other indicator in a synchronous manner, as described above.

,When the two gas tubes 8| and 96 have become conducting, they remain in that state and keep both the modulating oscillator and the obliterating oscillator out off until reset by actuation of the pushbutton switch IIIII. When this switch is momentarily depressed, the tubes 8| and '96 are short-circuited. Consequently, current ceases to flow between the cathodes and anodes thereof, permitting the control grids to resume control. If these control grids are sumciently negative, as in the absence of an input transient signal, they will regain control, and the tubes 8| and 93 will remain blocked or non-conducting.

Under some circumstances it may be desirable to maintain the modulated oscillator and obliterating oscillator. continuously cut off. This may be done by the switch 91 which, when closed, provides a permanent current path in parallel with the tubes 9| and 96, and thereby cuts ofi the two oscillators in the same manner as do tubes 33 and 9| when conducting. i

The circuit of the pickup head I9-9 is shown in Figure 3. The signal induced in 'the pickup head by the record on the tape is supplied successively through the four limiter amplifiers I29, I2I, I22 and I23. Tubes I26 and I2I are class C amplifiers, and suppress weak signals. This produces a clean base line in the oscilloscope image of the portion of the tape where the carrier frequency record was obliterated. Tubes I22 and I23 act as overloaded and clip of! and square the tops and bottoms of the alternations of the frequency-modulated signal. The output derived from tube H3 is therefore essentially a frequency-modulated square wave. This square wave is supplied to a differentiating circuit consisting of capacitor I2! and resistor I28 which converts the square wave into a succession of positive and negative pulses, each positive or negative pulse corresponding to the vertical increasing or decreasing part of the square wave. The time interval between these derived pulses will then vary in accordance with the original modulating signal controlling the frequency of the modulated oscillator.

These pulses are then supplied to the control grid of another grid-controlled gaseous rectifier tube I29, serving as a frequency discriminator. This tube I29 is biased to cut-off by voltage divider I30. The plate of tube I29 is connected to a source of positive potential through resistors I33 and I35. The junction of resistors I33 and I35 is then connected to ground through a capacitor I34. The cathode of tube I29 is connected to ground through a capacitor I3I shunted by a resistor I32,

In the absence of input pulses to the grid of tube I29, it remains non-conductive by virtue of the bias on the grid. Under these circumstances the capacitor I34 is charged to the full voltage of the positive supply source. When a positive pulse is impressed on the control grid of tube I29, tube I29 becomes conducting, and capacitor I34 substantially completely discharges into the capacitor I3 I, which has a much larger capacitance.

energy is supplied to capacitor I3I from the posi- 7 tive supply source, since the resistor I8! is designed to have a large value of resistance.

As soon as capacitor I" is substantially completely discharged, the current through tube M8 decreases to such a,low value that its grid resumes control, and'the tube is again blocked, whereupon capacitor I34 is again charged to the full positive potential of the source before the next positive pulse arrives. Capacitor ME ac cumulates this charging for a time long compared to the period of the modulated carrier, by virtue of the time constant of capacitor ItI and its shunt resistor I32. Each of the pulses of current supplied to capacitor I3I by the discharge oi capacitor I84 is therefore of the same magnitude and duration, determined by the capacitance of capacitor I36 and the voltage to which it is charged, and hence all such pulses have the same charge or integral of current. Accor the average voltage assumed by capacitor lot will depend upon the number of such equal energy pulses supplied to it per unit time. This n er of iii pulses per unit time, of course, depends on the number of positive pulses produced by the difierentiating circuit Ml, i233, which, in turn, cor= responds exactly to the frequency modulation impressed on the modulated oscillator carrier, which corresponds to the transient modulating signal. Thus, the average energy supplied to the capacitor I3I will correspond to the transient modulating signal.

In this way the voltage appearing across capacitor Iti is a faithful reproduction of the transient signal applied to the input circuit for the recording head. It will therefore be clear that the circuit oi tube lit provides a simple and eficient discriminator arrangement for ob= taining the modulating signal of a ireo uency= modulated wave.

The output of tube 629, derived as the voltage across capacitor iii, is supplied through a filter I36 to a cathode-follower amplifier circuit 539 and thence through a subsequent filter ass to output terminals Iii, which may be connected to the deflecting arrangement or the oscilloscope. The filters I36 and I40 are selectedto reove any undesired frequency components oi the signal, such as components of the frequency of the modu= lated oscillator, which might otherwise appear inthe circuit.

The discriminator arrangement just described is particularly useful where the modulating frequency is relatively low, such as of the order oi ten kilocycles per second or less.

For higher frequencies, the discriminator circuit shown in Figure 4 becomes very useful. This discrimmator is essentially a sell-recovering trig= ger circuit. The circuit consists of a first tube ltd having a load resistor idt which is coected to the control grid of a second tube Elli through the resistor 85d, which control grid is also con nected to a source or" negative potential through the resistor I55. The plate of tube 956 is con= nected to positive potential through the plate resistor I53 and also to the control grid of tube 85E through the parallel-connected resistor and capacitor I51, I53. The grid of tube itil is also connected to its cathode through a resistor I56. The input is supplied between the control grid of tube I and its cathode through a coupling capacitor I 59. The output is derived as the voltage-across a resistor I56 connecting a cathode of tube ISI to ground.

In the steady state or quiescent condition, the grid of tube I50 is positive and the tube is conducting. Tube ISI is thereby rendered non-conducting by the drop in potential produced by the current through plate resistor I52 of tube d. This quiescent condition is not-disturbed by any positive pulse :--==..1 on the grid of tube we, since this tube is already conducting. However, if a negative pulse is impressed upon the grid of tube 8%, this tube becomes less conducting, which, in turn, decreases the negative bias on the control grid oi tube iti, and tube lei becomes more conducting.

In the quiescent condition capacitor ass is charged substantially to the full potential of the positive source. When tube Iili becomes conducting, capacitor 856 discharges through resisters let, 5%, and the tube Hill. As this ca pacitor 058 discharges, the grid potential of tube itt rises, and a point is reached where tube B58 begins to conduct again. When tube let begins to conduct, the grid of tube iili is made more nesative, and tube idl is thereby cut oil, restoring the system to the quiescent state. During the time that tube ii was conducting, a definite vol age pulse appeared across the cathode resistor Itt which lasted for a definite length of time determined by the time constant of the circuit itl, its.

Thus, for every negative pulse (of whatever shape) impressed upon the input of tube E58, a rectangular pulse of current is passed through resistor tilt. The voltage across resistor Bill may then be applied to a suitable averaging circuit similar to capacitor till of Figure 3, whose output will then follow the frequency of the input pulses. Accordingly, this discriminator will uperate essentially in the same manner as that of Figure 3, to provide an output signal corresponding to the modulating signal impressed upon the modulated oscillator.

Since the discriminator of Figure 4 employs vacuum tubes, it does not suffer from difilculties caused by the listed deionization time of gaseous tubes, suchas used in Figure 3, so that the circuit oi Figure 4 is applicable to use with much higher frequencies than that of Figure 3.

Although the principles of the present invention have been described above as applied to one particular type of recording, they are obviously also applicable to many other types of recording, such as longitudinal magnetic recording, perpendicular magnetic recording, transverse magnetic recording, or the like.

It will be apparent to those skilled in the art that the novel principles of the invention dis closed here, in connection with specific exemplincations thereof, will suggest various other modincatlons and examples of the same. It is accorly desired that, in construing the breadth of the appended claim, it shall not be limited to the specific enemplincations of the invention described herein.

What is claimed as the invention is:

In an apparatus for recording and reproducing a transient signal: an endless magnetizable record medium, means for driving said record medium around a continuous loop at a given speed, recording head means in operative relation to said record medium for recording magnetic signals on moving portions of said record medium, a recording circuit including an oscillator connected to said recording head for supplying thereto mean-frequency oscillations in the absence of a transient signal to be recorded whereby said oscillations are continuously recorded upon said medium, means connected to said oscillator for frequency modulating said oscillations in accordance with the r to be recorded, a reproducing headin operative relation to said record medium for reproducing the signals recorded thereon, reproducing circuit means connected to said reproducing head and including means for amplifying and squaring said reproduced signals, said reproducing circuit means including means for diiferentiating said amplified and squared signal to establish a series of positive and negative pulses with time intervals between them which vary in accordance with the original modulating transient signal, frequency discriminator means connected to said differentiating means, and means for viewing said reproduced transient signal connected to said frequency discriminator means.

HARRY B. SHAPER. HARRY B. m.

REFERENCES crrnn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,262,407 Rath Nov. 11, 1941 2,334,726 Rankin Nov. 23, 1943 2,343,099 Usselman Feb. 29, 1944 2,370,133 Begun Feb. 27, 1945 2,378,383 Arndt June 19, 1945 2,378,388 Begun June 19, 1945 2,409,577 Maison Oct. 15, 1946 2,424,218 Begun July 22, 1947 

